(a) An order of disposition shall remain in force for an indefinite period until terminated by the court whenever it appears the purpose of the order has been achieved and it is in the child's best interest that he be discharged from further court jurisdiction.
(b) Unless sooner terminated by court order, all orders issued under this act shall terminate with respect to a child adjudicated in need of supervision when he reaches eighteen (18) years of age. If the child is still in the custody of the department upon attaining the age of eighteen (18) years, services may be provided on a case by case basis.
(c) Notwithstanding any other provision of this act, any placement of a child in need of supervision shall be reviewed by the court within six (6) months of the date of placement to determine if the placement is appropriate.